Fluorescence is a phenomenon where a material emits visible radiation when the material is excited by an external excitation source. A fluorescent lamp and a cathode ray tube each emit fluorescence. A material which emits fluorescence is called a phosphor. When light emitted by a phosphor lasts after cessation of excitation for a duration of time sufficient for the light to be perceived by the eye, i.e., about 0.1 second or longer, the light is called phosphorescence. A phosphor which has a long persistent phosphorescence lasting for several hours at room temperature is called a long-lasting phosphor or a light storage phosphor and it is to such materials that the present invention relates.
There are two types of prior art long-lasting phosphors materials namely a sulfide represented by ZnS:Cu and an Eu2+ activated alkaline-earth metal aluminate RAl2O4 (R being an alkaline-earth metal). The ZnS:Cu sulfide type of long-lasting phosphors have been used for several decades but they have the disadvantage that their after-glow lasts for a relatively short duration of time, usually, about three hours at the longest. Furthermore, this type of phosphor has a serious defect in that a decomposition reaction of ZnS+H.2O and Zn+H2S takes place under the coexistence of ultraviolet ray contained in the sunlight and moisture contained in the air. This renders the phosphor black in colour and, as a result, the after-glow characteristics significantly deteriorate within a relatively short period of time. For this reason, this type of phosphor has had only limited applications such as a luminous watch and a night-time display of a location in a house.
In order to improve the brightness and the length of after-glow, radioactive luminescent materials have been made, in which some radioactive elements such as CO, Ra, H3 were added. Such modifications can lead to the phosphor continuously giving out light. These phosphors have been used in aircraft dashboards and clocks. However, due to the possibility of radioactive contamination and the relatively high cost, the application of these phosphors has been severely limited.
Relatively Eu2+ activated alkaline metal aluminate long lasting phosphors have been developed (U.S. Pat. Nos. 5,376,303 and 5,424,006). These exhibit higher phosphorescence brightness, longer after-glow lifetime as well as better chemical durability and light resistance than the ZnS:Cu phosphors. These newly developed aluminate phosphors are anticipated to have wide applications such as signage, in addition to the existing applications for luminous watches and vehicle instruments.
Most of the known phosphor materials are subject to hydration when suspended in an aqueous solution and this causes them to suffer deterioration of performance. This in turn prevents the use of these materials in certain applications and in particular as pigments in water-based paints. Their use is limited to solvent-based solutions, resins, binders and heat formed compound plastics. However, their use in these applications is further limited to varying degrees from other negative characteristics. For example, it is not practical to obtain a stable suspension of known phosphor materials in solutions or emulsions because the phosphor material tends to settle. This is due to the high specific gravity of the phosphor materials which are typically in excess of 3 g/cm3. If settlement occurs with these phosphor materials the particles tend to solidify into a mass that is extremely difficult to break up and redisperse throughout the solution or emulsion. This characteristic of the phosphor materials dictates that the untreated phosphor materials be suspended in a given solution or emulsion on an in-situ basis immediately before use. In many applications this can be inconvenient.
A further problem arises with many known phosphor materials. They tend to be hard materials with various crystal structures. The hardness and crystal structures combine to make the phosphor materials highly abrasive. These characteristics are the largest contributors of frictional wear on processing machinery such as discs involving the use of phosphor materials with compound plastics and solutions. This frictional wear results in premature machinery wear and costly maintenance.
As well as the aluminate phosphors discussed above, silicate phosphors are also known. Furthermore, phosphors which contain a mixture of silicon and aluminium are known. Apart from limitations with colour and the length of the after-glow these known phosphors have other deficiencies, some of which have been described above.